Form-in-place EMI gaskets

ABSTRACT

A form in place conductive gasket is disclosed. The gasket, foamed, gelled or unfoamed is made of one or more elastomer resins, such as silicone urethane and/or thermoplastic block copolymers and is either filled with a conductive filler and lined onto a desired substrate or lined onto the substrate unfilled and then coated with a conductive outerlayer, such as a silver filled elastomer or a conductive flocked layer. A process and system for making the gaskets are also disclosed.

This application is a continuation of application Ser. No. 08/421,847,filed Apr. 14, 1995, now abandoned, which is a division of applicationSer. No. 08/119,403, filed Sep. 10, 1993, now abandoned.

The present invention relates to conductive form-in-place gaskets. Moreparticularly, it relates to conductive form-in-place gaskets having two,three or more axes of direction (e.g. X, Y and/or Z) and a process andsystem for making them.

BACKGROUND OF THE INVENTION

EMI shielding gaskets are used on electronic equipment to provideprotection against interference from electromagnetic energy, includingradio frequency interference (RFI) and more broadly all bands ofinterference commonly called electromagnetic interference (EMI). Theshielding has an electrically conductive element, be it a wire mesh,conductive filler or conductive plating, coating or fabric whichprevents external EMI from interfering with an electronic device and/orprotects other adjacent electronic devices from EMI emitted by anelectronic device.

Typically, EMI gaskets are prepared in one of three configurations:linear, die cut or compression molded. By linear, it is meant as anextrusion, molding, etc. of a defined, straight length. By die cut, itis meant that a gasket configuration is formed from a conductive sheetmaterial which is cut by a die to the desired shape, such as round,square, etc. By compression molded, it is meant that the gasketconfiguration is formed by placing uncured elastomer which may containconductive filler or a conductive mesh, into a specifically designedmold which is then subjected to compression (pressure) and then cured tocause the elastomer to assume the desired gasket configuration.

All three methods have disadvantages especially when used to formcomplex multidirectional or multiaxial gaskets, such as may occur indevices with a number of compartments that each need to be shielded,from each other as well as the external environment. Moreover, theproblems are even more critical on smaller devices, such as cellularphones, notebook computers and other hand held devices, where thediameter of the gasket becomes very small and the ability to manufactureand attach such gaskets securely becomes very difficult and laborintensive.

Using linear gasketing material to form complexmultiaxis/multidirectional gaskets (e.g. either x and y or in the x, yand z planes), is difficult, time consuming and costly. Each gasketportion must be hand cut and bonded to the adjacent portions of otherlinear gaskets and then bonded or secured in position upon thesubstrate.

Die cutting of conductive sheet stock will work in many instancesespecially in two plane (e.g. flat; x,y) applications, provided thateach portion of the gasket is wide enough and/or thick enough to be selfsupportive. Die cutting parts however results in significant waste ofthe sheet stock because the material is typically a crosslinked resinsuch as silicone or polyurethane. This is not acceptable as it drives upthe cost of such parts unacceptably. Further as diecutting is a roughprocess, the sheet stock needs to be fairly stiff and self supportivewhich is opposite that desired by the gasket user (i.e. soft andflexible).

Compression molding is slow and again generates scrap in the form offlash which must be removed. Further, each gasket design must use aspecifically designed mold, making the process expensive for all butlarge volume stock items.

A form in place EMI gasket and system for forming complexmultiaxis/multidirectional EMI gaskets which generates a minimum ofscrap, which forms the gasket in place and requires no special toolingis desired. The present invention provides such a system.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention relates to a form-in-place EMI gasket and a systemfor forming such a gasket using a table and/or dispenser that arecapable of moving in multiaxial directions relative to each other andthe substrate to be gasketed. The invention also relates to a process ofproviding a conductive elastomer, forming it in place along a desiredgasket configuration so as to create form-in-place EMI gasket.

An object of the present invention is an EMI shielded substratecomprising:

a.) a first electrically conductive substrate;

b.) a second electrically conductive substrate adjacent to the firstsubstrate;

c.) a formed in place electrically conductive gasket formed on andbonded to a predetermined portion of the first substrate so as toprovide an electrical connection and EMI shielding between the first andsecond substrates.

A further object of the present invention is an EMI shielded substratecomprising:

a.) a substrate having an electrically conductive surface;

b.) a cover for the substrate, the cover having an electricallyconductive surface which corresponds to and is in register with theconductive surface of the substrate; and

c.) a formed in place electrically conductive gasket formed on andbonded to a predetermined portion of the conductive surface of thesubstrate or cover so as to provide an electrical connection and EMIshielding between the substrate and cover upon the mating of the coverto the substrate.

Another object of the present invention is to an EMI gasket comprising acomposition formed of:

a.) a first component which is a primary polymer having end groups thatare capable of chemically reacting with each other in the presence ofmoisture to form a derivative polymer having a longer average chainlength than said primary polymer;

b.) a second component which is a noncross-linked elastomer that is notsubstantially chemically reactive with itself or with said firstcomponent; and

c.) a third component which is one or more electrically conductivefillers, wherein when said first, said second, said third components areintimately mixed, said composition, when maintained in the absence ofmoisture and other active hydrogen donor materials, being readilyextrudable and otherwise conventionally moldable thermoplasticcomposition but, upon exposure to moisture, becoming essentiallythermoset.

These and other objects of the invention will become clear from thefollowing description and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in planar view a preferred configuration of aform-in-place conductive EMI gasket, having a complex physical structurecomprising a series of elongated sections in the x, y axis, on a desiredsubstrate.

FIG. 2 shows in crosssection a preferred embodiment of the gasket ofFIG. 1.

FIG. 3 shows in planar view a preferred system for forming-in-place anEMI gasket, according to the present invention.

FIG. 4 shows another embodiment of a system of the present invention incrosssection.

FIG. 5 shows a crosssection of another preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a form-in-place EMI gasket.

Such gaskets are useful when positioned between two adjacent substratessuch as a boxed cover, so as to form an electrical bridge or continuitybetween the two substrates and thereby prevent or reduce the potentialfor EMI.

FIG. 1 shows a preferred embodiment of the present invention. The formin place conductive gasket 1 is mounted on a preselected section orsections of a substrate 2 which is to be shielded. In FIG. 1, thesubstrate 2 is a modular enclosure formed of two compartments 3, 4separated by a wall 5. Such an enclosure can be a cellular phonecarcass, a switching box, hard disk drive case, etc. As can be seen, thegasket 1 is formed in place along the edges of the enclosure which willmate with a cover (not shown).

FIG. 2 shows the conductive form in place gasket 11 in crosssection asmounted to the enclosure 12. In this embodiment, the area of theenclosure to be gasketed 13 has a shoulder 14 against which the gasketis formed. Other embodiments may not have the shoulder 14, being flatinstead, or may use other locating devices such as undercuts, dovetails,channels, etc. into which or against which the gasket may be formed.

The form in place gasket overcomes many of the problems of the priorapproaches. It eliminates the necessity to form the gasket and thenapply it in a separate step as occurs with the linear or die cutgaskets. It reduces waste as occurs with die cut or compression moldedgaskets by being a flashless or scrapless process. It is less laborintensive than linear or die cut gaskets as there is no hand assembly ofcomplex gasket shapes or mounting of the gaskets into place. Further,there is no need for the manufacture of specialized dies or molds whichare useful for only one gasket configuration. Instead, the gasket can beapplied to any substrate, in any configuration, easily and in a costeffective manner with a minimal investment for tooling. Moreover, withthe use of preprogrammable application equipment, one can store aninfinite number of gasket configurations which can be called up and usedquickly and repeatedly without the necessity to manufacture a specificdie or a mold.

Lastly, it allows for the exact placement of very small diameter gaskets(e.g. 0.010 inch diameter or less) which is difficult to achieve withall but compression molding.

The work "elastomeric" should be given its usual meaning given thepurpose for which the invention is intended. The elastomer bases used inthe invention can be thermosetting resins; e.g. resins which tocross-link and subsequently cure either at a critical temperature or inthe presence of a curing agent/catalyst such as peroxide,photoinitiator, moisture, etc. Any flexible thermosetting elastomer baseis suitable for use in the invention, such as EPDM copolymers, siliconerubbers, fluorosilicone rubbers, urethane rubbers, nitrile rubbers,butyl rubbers, and mixtures thereof.

Elastomeric thermoplastics may also be used. Thermoplastic rubbers, suchas various block copolymers (KRATON® rubber, NORPRENE® resin orSANTOPRENE® resin) are particularly useful. Omission of other elastomersis not meant to specifically exclude their use in the invention. Certainphysical or electrical requirements of the application for which thegaskets are intended may dictate that particular elastomericcompositions be used.

The selected material should be sufficiently viscous and/or form stableso that it does not slump, sag or run between the time of applicationand the time of curing. It may be in the form of a paste, a caulk, a gelor viscous fluid. Alternatively, when the material has a fast curingcycle or creates an initially stable material such as a gel or a skinnedor foam structure upon application, the material as applied can be arelatively thin or non-viscous fluid.

In addition, the selected resin should form a soft, resilient,compression set resistant gasket even with the addition of relativelyhighloadings of conductive fillers, if used.

Preferred elastomers that meet the requirements above include siliconerubbers, whether foamed or unfoamed; silicone gels, whether foamed orunfoamed, typically such gels are relative soft silicone rubber whichmay have been extended with oils or plasticizers or which are onlylightly crosslinked; polyurethanes, especially the prepolymer type ofurethane in which the prepolymer is capped or terminated with anisoacyanate group which when exposed to an activating agent (typically ahydroxy containing group), such as water, amines or alcohols cause theprepolymer to a crosslink; elastomeric thermoplastic rubbers such asDYNAFOAM® and NORPRENE® from Norton Co.; SANTOPRENE® resin fromMonsanto, and KRATON® rubber from Shell oil. These thermoplasticsgenerally comprise at least a block copolymer, such as SBS or SIS rubberwith or without other polymers (polyethylene, polystyrene, etc.) and/oroils or plasticizers. Additionally, various blends of the elastomers canbe used as well.

Such polymers are generally known and widely available, see e.g. U.S.Pat. Nos. 4,931,479, 4,643,924 and EP Patent Application 0326704A.

An EMI gasket can be formed of a composition as taught by EP PatentApplication 0326704A by using a two component polymer system, one whichis thermoplastic in the nature, the other being thermoset when exposedto moisture or active hydroxyl groups.

a first component which is a primary polymer having end groups that arecapable of chemically reacting with each other in the presence ofmoisture to form a derivative polymer having a longer average chainlength than said primary polymer, such as an isocyanide capped polyesterprepolymer;

a second component which is noncross-linked elastomer that is notsubstantially chemically reactive with itself or with said firstcomponent in the presence of moisture, such as a block copolymer e.g.styrene-butadiene-styrene block copolymers, and

a third component which is one or more electrically conductive fillerswherein the first, the second, and the third components are intimatelymixed, and the composition, when maintained in the absence of moistureand other active hydrogen donor materials, form a readily extrudable andotherwise conventionally moldable or coatable thermoplastic compositionbut, upon exposure to moisture, becoming essentially thermoset.

A preferred silicone gel is known as SYLGARD J27 (Parts A and B)available from Dow Corning Corporation. It can be mixed with one or moreconductive fillers to form an EMI material.

The gasket may be rendered electrically conductive either through theuse of a conductive filler incorporated into the elastomer base and/orthe use of an electrically conductive outer layer formed over a corewhich may be conductive or nonconductive.

The fillers that are used to impregnate elastomers to make themelectrically conductive are well-known in the art. Examples of thesefillers include but are not limited to electrically conductive noblemetal-based fillers such as pure silver; noble metal-plated noble metalssuch as silver plated gold; noble metal-plated non-noble metals such assilver plated copper, nickel or aluminum, for example, silver platedaluminum core particles or platnium plated copper particles; noble-metalplated glass, plastic or ceramics such as silver plated glassmicrospheres, noble-metal plated alumina or noble-metal plated plasticmicrospheres; noble-metal plated mica; and other such noble-metalconductive fillers. Non-noble metal-based materials are also suitable,including non-noble metal-plated non-noble metals such as copper-coatediron particles or nickel plated copper; non-noble metals, e.g. copper,aluminum, nickel, cobalt; and non-metal materials such as carbon blackand graphite combinations of the fillers to meet the desiredconductivity, hardness and other parameters desired for a particularapplication.

The shape and size of the electrically conductive fillers is notcritical to the present invention. The fillers may be of any shape thatis generally used in the manufacture of conductive materials, includingspherical, flake, platelet, irregular or fibrous (such as choppedfibers). In making gaskets in accordance with the invention it ispreferred that the particle shape be spherical, substantially sphericalor irregular. Flake or platelet shaped fillers are preferred when usedin forming an outer conductive coating for the foam-in-place gasket.

The particle size of the electrically conductive fillers can be withinthe range normally used for fillers in conductive materials. Generallythe particle size of the one or more fillers is from about 0.250 μ toabout 250 μ, preferably from about 0.250 μ to about 75 μ, and mostpreferably from about 0.250 μ to about 60 μ.

The amount, or loading, of the one or more electrically conductivefillers in the conductive elastomeric material used in the presentinvention can vary over a wide range, as long as the electricallyconductive filler is present in an amount sufficient to provide EMI/RFIshielding properties. Generally loading of the filler particles in theconductive elastomeric material is from about 10 to about 80 volumepercent, preferably from about 20 to about 66 volume percent.

When a conductive outer layer is used to provide the conductivity to thegasket, it may be in the form of a plating, a coating or a film. Platedlayers, such as silver plating, are not preferred as the platings tendto be rigid and crack under compression. Films, such as a conductivefilled polyethylene or polyimide, may be used.

Preferably, the outer conductive layer is some form of conductivecoating. More preferably it is a conductively filled resilient coating.Such coatings can and preferably are based upon the same elastomer resinthat is used to form the inner layer. Preferred coatings includesilicone, polyurethane, acrylic and epoxy resins filled with one or moreof the conductive fillers in the same size range and amounts asdescribed above.

Other fillers and ingredients may also be added to the elastomer base ifdesired. Such fillers include microwave absorbing materials, thermallyconductive fillers, inert or reinforcement fillers such as silicas andpigmentation fillers. Additionally, curing agents, cross linking agents,flame retardants, diluents, solvents or dispersion aids, etc., may beadded as is well known in the art to form the desired conductiveelastomeric material. In addition the elastomers may additionallycomprise other compounds, such as plasticizers, extender oils,softeners, tackifiers catalysts, blowing agents or other agents thatimpart desired properties to the cured gasket.

Typically, the gasket should have a SHORE A hardness (as measured byASTM standards) of between 5 and 60, preferably 5 and 50 and morepreferably 5 and 40. The properties of the gasket will vary dependingupon the resin chosen, whether it is foamed or not, the amount of fillercontained therein and the other constituents (oils, plasticizers,reinforcing fillers etc.) that may be added.

A typical form in place gasket should have a low closure force, e.g. aforce of less than about 5 pounds/linear inch preferably less than 3pounds, and more preferably less than 1 pound to deflect the gasketsufficiently to ensure proper electrical continuity between the twoadjacent substrates.

Before curing, the fluid-like gasket material should be capable of beingdistributed by automated equipment (if so desired) in diameters rangingfrom about 0.010 to 0.25 inch wide and aspect ratios as from about 0.25to 1 to about 3 to 1.

EMI Shielding effectiveness should be at least 10 dBs, preferably atleast 20 dBs over a range of frequencies from about 10 MHz to 10 GHz.More preferably, it should provide an EMI Shielding effectiveness offrom about 20 dBs to 80 dBs over a frequency range of from about 10 MHzto 10 GHz. Shielding effectiveness will of course vary with the amountof conductive material present, the deflection imposed upon the gasketand the test method used. All values above assume a typical loading ofconductive materials as referenced above, with at least 10% deflection,preferably 10 to 50% deflection and standard MIL spec. test procedures.

The process of applying such form-in-place conductive elastomerspreferably includes the use of automated equipment such as roboticapplicators, such as x-y, x-y-z and other such multiaxis or rotationaltype of applicators; hand applicators such as caulking guns; transferapplicators and other such processes.

Preferably, the process relates to the formation of an elastomer whichis capable of being formed in place, applying the elastomer to asubstrate along a predetermined pathway and curing the elastomer inplace.

If desired or required due to the elastomer resin selected and/or itsadhesion to a certain substrate, a bonding agent or primer may be used.

For example, some silicone compositions are known to have poor adhesionproperties, especially to metal substrates. A primer, such as a silane,silicate ester, a cyanurate or a silicone based adhesive may be used tocause the silicone composition to adhere to the metal substrate.

One preferred process is to use a stationary support or table to whichthe substrate to be gasketed is fixed in place. A movable applicator,such as a programmable x-y or x-y-z nozzle, which is connected to asupply of form in place elastomer, is positioned adjacent and preferablyabove the substrate and then caused to travel along a predeterminedpath, applying the elastomer to the portion of the substrate over whichit travels in a desired amount. The elastomer is then cured.

Alternatively, the nozzle may be stationary and the table may be causedto move in two (x-y), three (x-y-z) or more planes of movement.

In a further embodiment, both the nozzle and the table may move in oneor more planes relative to each other. One example is where the nozzlemoves in two planes (x-y) and is rotational as well and the table iscapable of vertical (z) movement.

Other variations and embodiments can be used as well.

A typical system for performing the process is shown in FIG. 3 wherein asupport platform or table 21 has a substrate to be gasketed 22 mountedupon it. An applicator, such as the lining dispensor 23 is located overthe platform 21. The dispensor is connected to a supply (not shown) ofform in place elastomer via a conduit 24.

The dispensor 23 is capable of moving in at least two planes of motionrelative to the platform, such as in the x and y axes. Preferably, it iscapable of moving in three planes of motion (x, y, z) and may also berotational so as to accommodate changes in the height or the angle ofthe substrate 21 over which it passes and applies the elastomer to forma gasket 25 at a desired position.

FIG. 4 shows another typical system in which the dispensor 26 and thetable 27 move relative to each other. Also in this instance, the nozzlehas two supply lines 28, 29 which allows for the use of two componentsystems such as urethanes or the introduction of a gaseous component(air, CO₂, nitrogen) into the elastomer just before application so as toform a foamed structure. The table 27 to which the substrate 30 ismounted is moved in one or more directions (x, y and/or z) by a drivemechanism as represented by box 31. The nozzle is moved via a similardrive mechanism 32.

One method of forming the form in place gasket is to mix a siliconerubber preferably in the form of a gel with conductive filler in anamount sufficient to provide EMI shielding. The mixture is then mixedwith additional silicone rubber and/or a curing agent or catalyst andthen added to a syringe applicator mounted on an x-y applicator arm. Thefluid-like gasket material is then distributed along a peripheral edgeof a substrate, such as a cellular phone housing where it cures inplace.

Another method is to form a nonconductive elastomer gasket layer, suchas by the process described above and then forming a conductive outerlayer over the nonconductive core via spraying, coating, painting ordipping a conductive outer layer onto the core. FIG. 5 shows such anembodiment. The gasket 40 is contained in a channel 41 formed in asubstrate 42. The inner layer 43 is covered at least partially with aconductive outer layer 44. The inner layer 43 is preferablynonconductive. However, if desired, it could be conductive, containinge.g. carbon black as a filler.

Alternatively, the nonconductive core can be coated with an adhesivelayer which is then flocked with conductive fibers, as taught by U.S.Pat. No. 5,115,104, which is incorporated herein in its entirety.

The gasket may be cured by any of the mechanisms commonly used for theselected polymer so long as the cure does not adversely affect the slumpproperties of the gasket between application and cure, and/or thephysical or electrical properties of the cured gasket.

Some elastomers such as prepolymer based polyurethanes are basicallyselfcuring in that once the reaction between the isocyanate group andhydroxy group begins, it typically continues until one or both groupsare expended.

Other elastomers, such as some silicone and thermoplastic rubbers usechemical curing agents such as peroxide, sulfur, zinc or amines and/orheat to crosslink and cure the resin.

Photocurable resins may also be used via the incorporation of a lightsensitive curing agent or photoinitiator which upon exposure to acertain wave length of light (UV, etc.) causes the resin to crosslinkand cure.

Some resins use heat to cure. In this instance in order to expedite thecure of the form in place gasket, one may warm the substrate before,during or after application (especially if it is metal) in order tohasten the cure and avoid problems of a cool substrate coating as a heatsink and drawing away heat from the resin. Alternatively, the resin canbe heated, such as in the nozzle just before application. The use of hotair or infrared heaters adjacent the substrate may also be used.

Hot melt resins such as those based upon KRATON® rubbers typically needto cool in order to set. Affirmative cooling of the substrate in thisinstance may be useful. Those hot melts which contain a crosslinkingagent may actually need to be kept at an elevated temperature (albeit itbelow the melting point of the resin) in order to crosslink.

The description above, and that which follows, is only meant to describeone particularly advantageous embodiment of the present invention and assuch is only meant to illustrate, not limit it.

EXAMPLE I

A conductive particle filled solid silicone form-in-place gasket wasmade of a two component silicone system, mixed in a ratio of 1 part A to1 part B.

Part A contained:

22.4 parts of silicone resin

77.6 parts of silver plated glass spheres, (avg. size 30-50 microns)

Part B contained:

22.3 of silicone resin

0.4 of a catalyst for the resin

77.3 of silver plated glass spheres

(30 to 50 microns average).

Part A and Part B were mixed separately by hand until each washomogenous. Then equal parts were added and mixed by hand untilhomogenous.

The mixed material was added to a 10 cc syringe with a needle tip of0.033 diameter. The syringe was mounted onto a dispensing head of aCAM-A-LOT Model 1818 x-y Positioning/Dispensing System. The material wasforced out of the syringe via air pressure of about 90 psi in programmedpatterns onto an aluminum flange (3 inches diameter, 0.25 inchthickness) which was mounted on a stationary table. The sample was curedin a hot air circulating oven for 60 minutes at 100° C.

The flange was cooled and placed in an Instron machine in a modifiedASTM D-575 Compression Testing fixture. Electrodes were placed into theflange opposite facing aluminum surface and the sample was compressed ata rate of 0.005 inch/minute to a total compression of 50% of theoriginal gasket height. During compression, stress, strain andresistivity values were recorded.

Following the compression testing, the flange was removed and bolted toa second flange (with no gasket) until the gasket was compressed to 50%its original height. The assembly was heated in a hot air circulatingoven for 22 hours at 85° C. The sample was removed, disassembled andallowed to cool and recover for 30 minutes. The gasket height wasremeasured and compression set was calculated as:

% set=

Original height - Final height

Original height - Deflected height

The results of those tests are shown in Table 1.

EXAMPLE 2

A conductive particle filled foamed silicone gasket was prepared andtested in the manner set forth in Example 1.

The Components of Example 2 were:

Part A -

21.6 Silicone RTV Foam

75.7 Silver Powder (325 mesh)

2.7 Toluene

Part B -

21.4 - Silicone RTV Foam

74.9 - Silver Powder

1.1 - Catalyst

2.6 - Toluene

The results are shown in Table 1.

EXAMPLE 3

A conductive coating over a nonconductive underlayer form-in-placegasket was prepared as follows:

The underlayer was prepared and applied as taught in Example 1. Theunderlayer or core comprised:

Part A -

84.2 - Silicone RTV Foam

10.5 - Cab-o-Sil (silica)

5.3 - Toluene

Part B -

80.8 -Silicone RTV Foam

4.0 - Catalyst (Sil gard 184B)

10.1 - Cab-o-Sil (silica)

5.1 - Toluene

After formation and cure, a conductive coating formed of silicone RTV,catalyst, solvent and silver coated glass conductive filler was appliedby brush to the outer surfaces of the underlayer.

The coating was comprised of two parts:

Part A -

11.5 -RTV Silicone

4.71 - Silver Powder

11.8 - Silver Flake

29.6 -Toluene

Part B -

100 RTV Silicone

The Parts were mixed in a ratio of 100 Part A to 1.21 Part B (byweight).

The results are shown in Table 1.

EXAMPLE 4

A stock, linear piece of conductive gasket, known as CHO-SEAL 1350gasket, available from Chomerics, Inc. and formed of a conductivelyfilled silicone rod, 0.060 inch in diameter was tested for compressionand resistivity values. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                             Electrical                                                  FORCE/ Resistance (ohms)     Com-                                            EX- DEFLECTION at stated Force/ pression                                      AM- (lb/inch) Deflection (lb/inch) Set                                      PLE  10%    20%    30%  40%  10%  20%  30%  40%  (%)                          ______________________________________                                        #1   .5     1.5    2.7  4.3  .060 .020 .011 .007 13                             #2 .7 2.2 4.0 6.2 .050 .010 .007 .002 50                                      #3 .3 1.3 2.5 4.0 .063 .024 .018 .016 41                                      #4 2.4 4.9 8.4 13.7 .037 .016 .010 .007 30                                  ______________________________________                                    

EXAMPLE 5

A conductive flocked form-in-place was formed of a urethane under layerapplied and cured as described in Example 1. The outer layer of theurethane under layer was coated with a flocking adhesive which in turnwas covered by a silver plated nylon flock as taught by U.S. Pat. No.5,115,104 which is incorporated by reference in its entirety. Theflocked gasket was placed in a hot air circulating oven for 10 minutesat 200° F. to cure the adhesive. The flocked gasket was found to provideEMI shielding over a wide range of frequencies.

EXAMPLE 6

A urethane form-in-place conductive gasket was prepared, assembled andtested according to the procedures of Example 1.

The gasket was formed of:

100 grams - urethane prepolymer

3 grams - activator

360 grams - silver powder

1.5 grams - silica (as a reinforcing filler)

The gasket was applied to and adhered to a substrate. The gasket wasfound to provide adequate EMI shielding over a wide range offrequencies.

What we claim:
 1. An EMI gasket comprising a composition formed of:a.) afirst component which is a primary polymer having end groups that arecapable of chemically reacting with each other in the presence ofmoisture to form a derivative polymer having a longer average chainlength than said primary polymer; b.) a second component which is anoncross-linked elastomer that is not substantially chemically reactivewith itself and with said first component; and c.) a third componentwhich is one or more electrically conductive fillers, wherein when saidfirst, said second, said third components are intimately mixed, saidcomposition, when maintained in the absence of moisture and other activehydrogen donor materials, being readily extrudable and otherwiseconventionally moldable thermoplastic composition but, upon exposure tomoisture, becoming essentially thermoset.
 2. The gasket of claim 1wherein the first component has a polyester backbone and isocyanate ortrialkylsilyl end groups, and the second component is selected from thegroup consisting of a styrene-isoprene-styrene block copolymer, astyrene-butadiene-styrene block copolymer, an isoprene homopolymer, apolyvinyl chloride, polyisobutylene, styrene-ethylene-butylene-styreneblock copolymer, and ethylene-propylene rubber and the third componentis selected from the group consisting of noble metal fillers; noblemetal-plated noble metals; noble metal-plated non-noble metals;noble-metal plated glass, plastic or ceramics; noble-metal plated mica;non-noble metals, non-noble metal-metal plated non-noble metals; carbonblack; graphite and combinations thereof.
 3. The gasket according toclaim 1, wherein the gasket has a shore A hardness of from about 5 toabout
 50. 4. The gasket according to claim 1, wherein the gasket has ashore A hardness of from about 5 to about
 40. 5. The gasket according toclaim 1, wherein the gasket has a diameter of from about 0.01 inches toabout 0.025 inches.
 6. The gasket according to claim 1, wherein thegasket has an electrical resistance of from about 0.005 ohms to about0.1 ohms, a force/deflection value of from about 0.2 pounds/inch toabout 15 pounds/inch, and a shore A hardness of from about 5 to about60.